The third complementarity determining region of the immunoglobulin heavy chain-HCDR3-lies at the center of the antigen-binding site and typically determines antibody specificity. The DH gene segments that compose the core of HCDR3 are potentially translatable in six reading frames, each with a characteristic hydropathicity signature-charged, hydrophobic, or hydrophilic. However, in virtually all species the final HCDR3 repertoire is enriched for neutral, hydrophilic sequence. Charged or hydrophobic HCDR3 intervals are common only in exceptional circumstances (e.g. pathogenic anti-DNA antibodies). Our hypothesis holds that charged or hydrophobic HCDR3 domains are "forbidden" because they are more likely to generate self- reactive antibodies and thus help "trigger" autoimmune disease in susceptible organisms. Through gene targeting, we are generating mice wherein the antibody repertoire has been globally altered to produce HCDR3 intervals encoded by either charged or hydrophobic DH reading frames. These altered repertoires contain native charged or hydrophobic HCDR3 sequence that, although rare in typical antibodies, is more common in some autoimmune prone strains of mice. Preliminary studies indicate that B cells expressing antibodies with charged HCDR3 intervals can be generated in high numbers, but are selected against in the periphery. We propose that this peripheral loss of B cells is due to self-reactivity. Mice that bear genes that reduce the barrier to the formation of autoantibodies may enable B cells expressing antibodies with altered HCDR3 hydropathicity to survive. Antibodies with charged or hydrophobic HCDR3 intervals are proposed to be more likely to "trigger" autoimmune disease. In order to test this hypothesis, we will introduce mutant DH loci into mice bearing defined susceptibility loci for autoimmune disease, e.g. C57BL/6 bearing the NZM derived sle1, sle2, and sle3 loci. The role of these susceptibility loci in facilitating the development of B cells with charged or hydrophobic HCDR3 intervals will be determined. To determine whether alteration of the repertoire can affect the ability of the mice to respond to normal stimuli, the mice will be challenged with T-independent and T-dependent antigens, and with Strep. pneumoniae and influenza virus. Finally, the role of common and early expression of antibodies bearing charged or hydrophobic HCDR3 intervals on the development of autoimmune disease will be examined. These studies should either confirm or lay to rest the theory that use of alternative DH reading frames in the antibody repertoire promotes the production of autoantibodies. If, as we expect, the answer is yes; then the proposed experiments have the further potential of helping us understand the role that sle1, sle2, and sle3 may have in voiding normal regulation of abnormal antibody repertoires.